1. Field of the Invention
This invention relates generally to a method and apparatus for bonding a bare semiconductor chip or die to a leadframe and, more specifically, to a method and apparatus for injecting and applying an atomized adhesive to a semiconductor device component.
2. State of the Art
A leadframe is basically the backbone of a typical molded plastic package. Leadframes serve first as a die support fixture during the assembly process and are subsequently electrically connected to the die bond pads after die-attach, as by wire bonding. After transfer molding, the leadframe becomes an integral part of the package. Generally, leadframes are fabricated from a strip of sheet metal by stamping or chemical milling (etching) and are made from various materials selected for cost, ease of fabrication, and various functional (mechanical and electrical) requirements. Typical leadframe materials generally include nickel-iron, clad strip, or copper-based alloys.
An important feature of a leadframe is its ability to channel heat from the chip to the exterior of the package, which ability is dependent on the thermal conductivity of the leadframe material. Copper alloy leadframes are desirable from this standpoint. However, copper alloy leadframes also have high thermal-expansion rates (based on coefficients of thermal expansion) in comparison to silicon, but nearly match the expansion rates of low-stress molding compounds. Consequently, the chip-bonding material, that is, the die-attach material used to bond the chip to the leadframe, must be carefully selected. For example, silicon/gold eutectic bonding cannot be used with copper frames because its high elastic modulus couples thermally induced bending stresses generated by leadframe expansion to the silicon of the die, significantly increasing the potential for die fracture. As a result, silver-filled epoxies and polyimide die-attach adhesives have been developed that are flexible enough to accommodate the stress initiated by an expanding copper leadframe so that the die is not subjected to strain.
Leadframes are typically supplied in multi-frame strips designed with automated assembly, wire-bond and packaging system in mind. As such, tooling or indexing holes are located along the leadframe strip edges to mate with transfer-mechanism elements and alignment pins. Such pins are typically part of the assembly equipment, including die bonders, wire bonders, molds, auto-inspection stations, trim and form equipment, and marking machines.
As noted above, both silicon/gold eutectic, as well as adhesive bonding materials, have been used to bond the die to the leadframe. For silicon/gold eutectic bonding, the operation typically begins by indexing dice on a bonding machine to a heated die support platform. Leadframes are then fed from magazines along a track to a heater block. A small square of silicon/gold alloy (typically 6% Si, 94% Au) is cut from a feed ribbon and transferred to the die support platform, also called a die-attach paddle, tab or island. Die and eutectic are then scrubbed together, forming a hard alloy bond. The heater temperature is approximately 420xc2x0 C. and the total cycle time for eutectic bonding is about 6 to 8 seconds.
Adhesive bonding is faster than eutectic, with a cycle time of about 2 seconds. Typical feed mechanisms for polymer bonding machines are the same as eutectic bonders. The leadframes, however, are usually not heated. Silver or gold-filled epoxy or polyimide adhesive paste is transferred to the die support platform by a print head, and a die is pressed into the paste immediately after printing. Die bonding adhesives manufactured by Epotek, Amicon and Ablestick are typically electrically conductive, have maximum cure temperatures up to 275xc2x0 C., and have lap shear strengths up to 2.11 kg/mm2.
The die bonded leadframe strips are subsequently loaded into transport magazines. Eutectic-bonded frames go directly to a wire bonding station, while magazines containing adhesive-bonded frames are routed to ovens for curing. The curing atmosphere is typically dry nitrogen, and usually requires one hour at 150xc2x0 C. to cure, followed by 30 minutes at 275xc2x0 C. for polyimide adhesives.
A large number of polymers, copolymers, and polymer blends have been developed in the past several decades with the aim of joining composites. Epoxies, however, have been the primary material used to bond laminates, and such adhesives have been the principal bonding agent for printed circuits. Due to their ability to react with many types of compounds and to enter solid solution with a variety of modifiers, epoxies can be formulated to meet most requirements that do not exceed their use temperature of 125xc2x0 to 150xc2x0 C. For thermoplastic adhesives requiring the adhesive material to be heated to temperatures around 200-300xc2x0 C., up to three hours of cure time may be required to remove any solvents. Epoxies, moreover, are generally very viscous and consequently somewhat difficult to handle and apply.
Acrylics have been customarily used as adhesives for polyimides requiring temperatures higher than the maximum use temperatures of epoxies. Polyimidesiloxane hybrids have also been used that have superior thermal resistance and good compatibility for this purpose. For applications requiring the highest temperature or the most demanding dielectric requirements, polyperfluorocarbons may be used. Perfluorethylenepropylene copolymer films also exhibit suitable adhesion if the adhered surface is prepared properly.
Typically, rough or absorbent adherends readily bond together. Smooth, impervious materials, however, are much more difficult to adhere, and these are, more typical of printed-circuit substrates. Smooth surfaces, even if clean, usually cannot be bonded unless roughening or chemical treatment of the adherend surface precedes adhesive application. Metal surfaces too smooth to be bonded can be roughened by abrasion, but more frequently unalleviated metal surfaces as on power or ground planes are prepared for bonding by chemical modification. Alkaline oxidation of copper provides an instant oxide surface more polar and irregular than the original. Chelates such as benzotriazoles bond well to the oxide that is always formed on copper, and can be stable to 200xc2x0 C. These chelates allow good adhesion to organics, have high cohesive strength, and serve as corrosion inhibitors when used as coatings.
Typical prior art devices apply die bonding adhesives by rolling, stamping, tape application, or spraying. That is, an adhesive-bearing roller or a stamp may be used to apply the adhesive to the die-attach area of the leadframe. Similarly, a mask may be placed over the portion of the leadframe where adhesive is not desired and a spray nozzle may spray the masked leadframes with the desired adhesive. Other methods have also been developed,such as that disclosed in U.S. Pat. No. 5,286,679, in which a patterned adhesive layer is deposited by hot or cold screen printing the adhesive, by photopatterning a photosensitive adhesive, or by utilizing a resist method of etch back. Adhesive-coated tapes have also been utilized to bond the die to the leadframe, as disclosed in U.S. Pat. No. 5,304,842, as well as adhesive patches applied from tape carriers, as disclosed in U.S. Pat. No. 5,448,450.
The prior art systems, however, have several notable disadvantages. For example, such systems may not evenly distribute the adhesive across a die-attach paddle of the leadframe. Moreover, application of an adhesive having large particle sizes may cause voids or bubbles to form on the leadframe. Other adhesive applicator apparatuses do not draw the adhesive evenly onto the die-attach paddle or the lead fingers (in the case of a leads-over-chip, or LOC, die-attach) and may have relatively slow cycle times. In addition, many prior art systems do not fully enclose or focus the adhesive application process to substantially reduce, if not substantially eliminate contamination of equipment components and leadframe areas where adhesive is not desired. Prior art systems may also require an additional, preliminary masking step in the manufacturing process to ensure that the adhesive is only applied to desired locations. Finally, many prior art systems may require an extended curing cycle in an oven, for example, to cure the adhesive or epoxy. In some systems, after die-attach, an optical alignment system checks to ensure that the die has been properly placed, translationally and rotationally, on the leadframe. Between die-attach and wire bonding, however, dies requiring an extended epoxy cure cycle often have their orientation or placement distorted as the adhesive cures, adversely affecting the wire-bond operation and, thus, quality and reliability of the product.
It is also desirable in some circumstances to apply adhesive to a wafer surface for so-called leads-over-chip or LOC mounting of a die under the leads of a leadframe without a die-attach paddle. Currently, screen-printing is employed to selectively coat the wafer. Alternatively, a spin-on process may be employed to coat the entire wafer surface, followed by selective etching away of the adhesive in undesired locations. Adhesively-coated tapes are also employed for LOC die-attach. Such processes, as with those previously described, leave much to be desired in terms of process time, ease of use, and resulting product quality.
Thus, it would be advantageous to provide an apparatus for applying an atomized adhesive to a semiconductor device component for die bonding that encloses the adhesive application process, evenly draws the adhesive onto the die-attach paddle and removes excess adhesive, automatically masks or shields each component to protect certain areas from being sprayed with adhesive, and has a relatively fast cycle time.
Accordingly, the present invention provides a method and apparatus for the precise application of a die bonding adhesive to a semiconductor device component using an injection nozzle to atomize the die-attach adhesive.
In one preferred embodiment, the apparatus includes a containment hood with an associated spray nozzle in communication with an adhesive reservoir and positioned above an adhesive application location or target area. An air purge may be incorporated within the hood to drive the adhesive onto the die-attach paddle area for a further reduction in cycle time. In addition, the plenum of a vacuum or negative-pressure system may also be positioned on the opposite side of (usually below) the adhesive application location from the injection nozzle to help draw the adhesive onto the die paddle and carry away any excess atomized adhesive. The hood and/or lower vacuum system may be connected to a cam system or other mechanism as known in the art that allows for simultaneous opening and closing of the hood and/or vacuum plenum around the target area to allow indexing of semiconductor device components through the apparatus. Further, the apparatus preferably utilizes flexible seals on the edges of the hood and lower vacuum assembly to contain the adhesive application process to the area where adhesive deposition is desired. When the apparatus is used to apply adhesive to leadframes, the seals may contact the dam bars and side rails of the leadframe to provide the maximum amount of vacuum and adhesive leakage from the target area.
In one aspect of a preferred embodiment, the apparatus has a shutter-type mechanism that opens an aperture above the leadframe, allowing a vacuum to draw the atomized adhesive mixture evenly onto the die-attach paddle for a specified cycle time. Preferably, the adhesive is atomized to a particle size of 50-100 xcexcm. The aperture is sized and shaped to cover various portions of the leadframe to shield those portions and prevent application of adhesive thereto.
After a leadframe has been fed into the target area, the die-attach paddle will be located under the hood. The leadframe may be optically and/or mechanically aligned by methods known in the art. A mechanical alignment system employing the leadframe strip indexing holes is preferred for simplicity and substantial immunity to contamination by adhesive. The cam system then closes and seals the hood and the vacuum assembly plenum about the leadframe. A shutter positioned above the leadframe opens and a mask covers the surrounding area of the leadframe that does not require any adhesive. The injection nozzle inside the hood injects, under high pressure, a selected quantity of adhesive that atomizes and mixes with air provided to the containment hood from an exterior source, which is normally outside of the clean room where the adhesive application apparatus is located. The vacuum below the leadframe pulls the atomized adhesive particles downwardly and deposits them evenly onto the die-attach paddle.
Using an electronic or mechanical timing device, the shutter may be released to close once a predefined amount of adhesive has been injected. The shutter, as well as dictating exposure of the leadframe to the adhesive mist, also substantially prevents adhesive dripping. For faster cycle times, an additional air purge within or above the hood and behind or above the atomizer nozzle can be utilized to apply a small air burst or puff to help accelerate and direct the adhesive toward the leadframe. After the adhesive has been applied, the shutter closes, the cam system opens the hood, and a feed mechanism indexes the leadframe toward the next position. By design, the apparatus of the invention can be placed in an otherwise conventional die-attach sequence in place of, for example, a conventional epoxy stamp.
The hood, shutter mechanism, and vacuum system preferably incorporate one or more drainage channels for collection and removal of excess adhesive. If the volume of air under the hood becomes oversaturated with adhesive, for example, causing the adhesive to drip, the drainage channels can collect the adhesive and recycle it to the adhesive reservoir. Cleaning the system of the invention should be less frequent than other spray-type systems because the excess adhesive is pulled into a recirculating or disposal system.
In another preferred embodiment, the hood, vacuum system, and shutter can be modified to simultaneously accommodate multiple leadframes, if desired, such that the adhesive could be applied across all desired leadframe areas of a leadframe strip simultaneously in one cycle.
Preferably, the adhesive employed with the application system of the invention should have characteristics similar to epoxies currently available. That is, the adhesive must be conductive and may be colored or otherwise detectible by an automatic imaging system. Further, the adhesive should be provided in liquid form or liquefied prior to atomization by heating to a desired viscosity for proper atomization. Adhesives having a relatively low solvent content would have the effect of greatly reducing the amount of cure time. Moreover, use of such an adhesive formulated to have a cure time of 10 seconds or less after application to the die-attach paddle could eliminate the entire oven-cure process and provide for immediate, aligned attachment and immobilization of the die on the leadframe. Moreover, an adhesive having these characteristics could reduce total process time by 2-3 hours by eliminating elevated temperature cure time in an oven. In selecting such an adhesive, the effects of temperature encountered during a transfer molding or other encapsulation process should also be considered.
The apparatus and method herein described will prevent excess adhesive from being deposited on the leadframe, provide more precise and evenly distributed adhesive application, increase the reliability of the process, and produce faster cycle times than prior art systems. Moreover, an increased adhesion area of adhesive will result due to the small particle size of the atomized adhesive, eliminating uncoated inter-particle spaces and filling any voids or vugs on the surface of the leadframe or other semiconductor device component. The potential for bubbles or voids within the adhesive layer will also be substantially reduced as compared to application of thicker, more viscous epoxies used in the art.
In another preferred embodiment, an adhesive spray may be aimed toward a wafer instead of being misted and then drawn toward the target by air flow. For example, a wafer is loaded onto a working platen by a mechanical loading arm. Once on the platen, the wafer is held in place by a vacuum and aligned by an imaging alignment system using fiducial marks as known in the art. After alignment, a stencil is automatically placed on the surface of the wafer to mask the areas where adhesive is not desired. The hood then seals around the wafer periphery and a spray nozzle aimed toward the wafer deposits an even coating of adhesive. The spray nozzle may be translatable along a spray path over the wafer, or multiple nozzles used on a fixed or translatable spray bar. Multiple passes of the nozzle(s) could thus be made depending on the spray area, adhesive volume delivered per pass, and adhesive thickness desired. The stencil is then removed and the wafer unloaded by a mechanical arm. The process is then repeated for other wafers. When sufficient adhesive material accumulates on the stencil, the stencil is removed and replaced with a clean one. Alternatively, the stencil may be cleaned after each wafer, or the stencil fabricated from a disposable material and discarded after one or more uses.
Similarly, when spraying leadframes according to the above-described embodiment, a mechanical arm or conveyor loads a leadframe or strip of frames to an adhesive application location or target area. When the leadframe or strip is properly positioned and masked by stencil, if desired, a sensor stops leadframe movement and the spray nozzle or nozzles deposit the adhesive. When spraying is complete, the stencil is lifted, and the leadframe or strip is removed by the mechanical arm or moved along by the conveyor to the pick-and-place die-attach station. The invention thus reduces cycle time through full automation of the equipment, reduction in the number of cleaning cycles, and reduced adhesive application and die-attach time. The system has utility with all types of leadframes, including those with a die-attach paddle as well as LOC leadframes, for which it is particularly well-suited.
In either system described above, the machine parameters such as spray pressure, temperature, nozzle height, nozzle speed, nozzle type, nozzle aperture size, spray pattern, cycle time, and (where applicable) vacuum or negative pressure below the target area would be dependent on the adhesive material used and desired thickness of the adhesive. It should be noted that references to adhesives and other bonding agents in the specification and claims comprise adhesives such as epoxies, as well as all other sprayable bonding materials as known in the art.
In addition, the invention disclosed herein has equal application and utility with regard to the coating of leadframes, leadframe strips, conductor-carrying boards and other substrates, semiconductor wafers, partial wafers and singulated dice, although the latter may not be practical for high-volume operations. Thus, the use of the term xe2x80x9csemiconductor device componentxe2x80x9d as used in the specification and claims connotes any of these items, and contemplates the application of adhesives thereto.